1. Field of the Invention
The field of the invention relates to electrode surface coating and electroplating processes for deposition of surface coating.
2. Description of Related Art
Platinum has often been used as a material for electrodes in corrosive environments such as the human body due to its superior electrical characteristics, biocompatibility and stability. Platinum has many desirable qualities for use as an electrode for electrical stimulation of body tissue. Since platinum has a smooth surface and its surface area is limited by the geometry of the electrode, it is not efficient for transferring electrical charge. The platinum with a smooth surface is hereinafter called “shiny platinum”.
Electrodes for stimulating body tissue by electrical stimulation are known in great variety. For the utility of an implantable stimulation or sensing electrode—especially one intended for long-term use in a tissue stimulator with a non-renewable energy source and that, therefore, must require minimal energy—a high electrode capacitance and correspondingly low electrical impedance is of great importance. Furthermore, without sufficiently low impedance, a large voltage may cause polarization of both the electrode and the tissue to which the electrode is attached forming possibly harmful byproducts, degrading the electrode and damaging the tissue.
Because the ability of an electrode to transfer current is proportional to the surface area of the electrode and because small electrodes are necessary to create a precise signal to stimulate a single nerve or small group of nerves, many in the art have attempted to improve the ability of an electrode to transfer charge by increasing the surface area of the electrode without increasing the size of the electrode.
One approach to increase the surface area of a platinum electrode without increasing the electrode size and therefore to improve the ability of the electrode to transfer charge is to electroplate platinum rapidly such that the platinum molecules do not have time to arrange into a smooth, shiny surface. The rapid electroplating forms a platinum surface which is commonly known as “platinum black”. Platinum black has a porous and rough surface which is less dense and less reflective than shiny platinum. U.S. Pat. No. 4,240,878 to Carter describes a method of plating platinum black on tantalum.
The US patent application No. 2003/0233134 “Biocompatible Bonding Method and Electronics Package Suitable for Implantation” to Robert J. Greenberg et al. filed Sep. 6, 2002, the disclosure of which is incorporated herein by reference, discloses a device comprising a substrate at least a portion of which is electrically conductive; a flexible assembly; and at least one deposited rivet that bonds said substrate and said flexible assembly together.
The US patent application No. 2004/0220652 “Adherent Metal Oxide Coating Forming a High Surface Area Electrode” to Dao Min Zhou et al. filed Nov. 4, 2004, the disclosure of which is incorporated herein by reference, discloses an implantable electrode comprising: a roughened conductive substrate; and a rough surface coating on said substrate.
Platinum black is more porous and less dense than shiny platinum. Platinum black has weak structural and physical strength and is therefore not suitable for applications where the electrode is subject to even minimal physical stresses. Platinum black also requires additives such as lead to promote rapid plating. Lead, however, is a neurotoxin and cannot be used in biological systems. Finally, due to platinum black's weak structure, the plating thickness is quite limited. Thick layers of platinum black simply fall apart.
For the foregoing reasons there is a need for an improved platinum surface coating and process for electroplating the surface to obtain an increased surface area for a given geometry and at the same time the coating is structurally strong enough to be used in applications where the platinum surface coating is subject to physical stresses.